Oligostyrenes have long caught the attention of researchers in this field. Styrene is a readily available starting material and can be polymerized to form telomeric, oligomeric and low molecular weight polymeric distributions of polystyrene chains. Such polymerization reactions include radical, cationic and anionic mechanisms with and without chain transfer. However despite the attention and effort given this broad class of low molecular weight aromatic substrates, no commercially successful brominated ultra-low molecular weight oligostyrene or styrene based telomer having a bromine content above 71 wt % and a desirable combination of properties such as (i) initial color as measured by total solution color and Hunter color yellowness index; (ii) high thermal stability as measured by thermogravimetric analysis (TGA), thermal HBr, and thermal color; and (iii) optimal glass transition temperatures (Tg) has found success in the market place. This is surprising in that the potential of a useful low molecular weight brominated oligostyrene has been known since at least 1992 (see U.S. Pat. No. 5,112,898).
It is clear from the patent literature that low molecular weight oligostyrenes can be brominated either with bromine of bromine chloride to provide brominated oligostyrenes with bromine content of less than about 71 wt % bromine. Beyond that, the details provided are quite limited, hence the quality and utility of such brominated oligostyrene compositions as broad spectrum flame retardants is left to inference only, and have never been proven in a marketable product. Given that the key properties that delineate a useful flame retardant were omitted in the referenced patents, it may be reasonable to infer that the compositions thus produced were lacking key performance characteristics or economic advantages to warrant a commercial venture. For example, the low molecular weight oligostyrenes known heretofore are typically too expensive to be of commercial interest, or are in need of additional processing to gain some improvement in their properties. Moreover, brominated low molecular weight oligostyrenes known heretofore require additives to improve their thermal stability and/or require isolation from commercially unattractive processes.
Particularly lacking are the experimental details regarding the formation of the oligostyrene substrates. The few details that are provided point to oligostyrene distribution produce either neat or in solution using radical initiators at moderately high temperature or no initiator at extremely elevated temperatures. In addition to lacking the experimental details of their formation, none of the patents or any of the patents referenced with in those patents adequately describes the molecular weight distribution of the oligostyrene substrates that were brominated. Generally speaking all that is reported is one and only one of the following molecular weight parameters: DPn; Mp; Mn; or Mw. It can be accepted, as an axiom that the performance characteristics of any polymeric and or oligomeric distribution are highly dependent upon many features among these is molecular architecture, microstructure, and molecular weight distribution. Each of these parameters is dependent upon the chemistries and conditions used to form such distributions. To liken one oligostyrene to another oligostyrene based on a similarity of any one or even the equivalency of all four aforementioned molecular weight distribution parameters is a fallacy. As is written in U.S. Pat. No. 5,687,090: “A polymer component is an ensemble of molecules whose properties are represented not as single values, but instead as property distributions. The properties of interest include the polymer molecular weight or chain length, copolymer composition, type and number of branches, type and number of end-groups, etc”
Heretofore, suggestions have been made to utilize various ultra low molecular weight oligostyrenes formed from anionic polystyrene (APS) mechanisms. However, the cost of an analogous (APS) oligostyrenes having a DPn in the range of interest for use in forming polymeric brominated flame retardants for high impact polystyrene is prohibitive. The high cost is necessitated by the use of a stoichiometric amount of an organolithium polymerization initiator.
Setting aside for but a moment the unfavorable economic factors associated with an APS based oligostyrene having a DPn in the range of 3 to about 20, there have been two literature reports of the synthesis of benzyl oligostyrene, (“benzyl-OS”) via living anionic oligomerization using stoichiometric benzyl lithium formed from the reaction of butyllithium with toluene using N,N,N′N′-tetramethylethylenediamine, TMEDA: (1) Tsukahara, Y. et. al. Polymer Journal 1994 26 1013; and (2) Nakamura, Y. et. al. Macromolecules 2005, 38, 4432. The structure of these benzyl oligostyrenes is represented by the formula:

Between the two papers ten different benzyl-OS products were reported with Mw and PD as follows: Benzyl-OS #1 1670, 1.15; #2 6710, 1.18; #3 15550, 1.13; #4 13400, 1.10; #5 16800, 10.7; #6 710, 1.13; #7 1870, 1.06; #8 3320, 1.03; #9 3440, 1.01; #10 7650, 1.08. Mn can be calculated from polydispersity but unfortunately the value of Mz was not reported in either paper for any of the distributions; in as much as APS chemistry is well known to produce near Gaussian shaped distributions. Hence Mn and Mz are symmetrically displaced about the central tendency or mean Mw and Mz does not deviate significantly from Mw.
Of these 10 structures, benzyl-OS #6 would appear to have a number average degree of polymerization (DPn) in the range of interest for forming polymeric brominated flame retardants for use in high impact polystyrene applications. This product having a normal molecular weight distribution is represented by FIG. 1. However, as noted above, achieving this normal weight distribution is prohibitively expensive for flame retardant applications. The 710 Mw APS benzyl-OS #6 referred to above requires 1 mole of a costly organolithium reagent for every 6 moles of styrene charged, and hence is not economically feasible as a suitable low molecular weight styrenic distribution substrate for formation of flame retardants and in turn use in flame retardant applications. Moreover, use of this 1:6 ratio of organolithium reagent to styrene necessitates use of a highly diluted reaction medium to maintain the solubility of the growing living polymer chains throughout the course formation of 710 Mw, APS benzyl-OS #6. Hence, the process efficiency is very low and incurs very high capital and operating costs, in addition to the very high raw material costs.
A feature of this invention is that it is now possible to produce on a commercially attractive economical basis, similarly populated molecular weight distributions with molecular weight parameters that approximate but are different from the normal molecular weight distribution of benzyl-OS #6.
It is important at this point to make further distinction between anionic oligostyrene distributions and other anionic styrenic distributions. For the purpose of clarification of this invention we utilize three different kinetic classifications as follows:    1. Oligomerization (No Chain Transfer, Mn≈Mcalc), ktr=0.    2. Effective Chain Transfer (Reduction in Molecular Weight, Mn<Mcalc) ki≈kp>ktr     3. Telomerization (Large Reduction in Molecular Weight, Mn<<Mcalc) ki≈kP<ktr. where Mn is the number average molecular weight, Mcalc is the calculated molecular weight in the absence of chain transfer and is given by the expression: Mcalc≈(moles monomer/moles initiator)·(molecular weight of monomer), ki is the rate of initiation, ktr is the rate constant for chain transfer from the chain transfer agent to a growing chain and kp is the polymerization rate constant.An APS based oligostyrene distribution is one that formed under kinetic condition 1 above. In contrast, for a styrenic distribution formed from an anionic chain transfer process the kinetic condition described by either 2 or 3 above exists during its formation. Such a reaction will be referred to as anionic chain transfer styrenic reaction (sometimes designated hereinafter as “ACTSR”). A styrenic distribution formed from such a process will be referred to as an anionic chain transfer styrenic reaction (sometimes designated hereinafter as “ACTSR distribution”). These definitions are adapted from Rudin's (A. Rudin, The Elements of Polymer Science and Engineering, Academic Press, Orlando, 1982, pp. 3 and 212) definition of an oligomer and regarding chain transfer radical polymerizations. An ACTSR distribution formed under kinetic condition 3 is an anionic chain transfer styrenic telomer (sometimes referred to hereinafter as “ACTST distribution”). A telomer distribution of this invention is formed via anionic chain transfer using toluene as the chain transfer agent, hence such a distribution will be referred to as a toluene styrenic telomer distribution, (sometimes referred to hereinafter as “TSTD”). Reference is made to analogous ethylbenzene distributions; such a distribution formed from ethylbenzene will sometimes be referred to hereinafter as an “EBSTD”.
The Nakamura and Tsukahara processes clearly yield oligostyrene distributions in that the process is run under such conditions (condition 1 above) in which no chain transfer can occur. Examples of ACTSR distributions formed under effective chain transfer conditions (condition 2 above) can be found in both Gatzke's report (A. L. Gatzke, J. Polymer Science, Part A-1, volume 7, pages 2281-2292, (1969) and in published European patent application EP 0 741 147 A1. Additionally, ethylbenzene styrenic telomer distributions (kinetic condition 3), EBSTD, have been reported in EP 0 741 147 A1.
EP 0 741 147 A1 is of interest in that it contains a discussion of some of the important reaction parameters relevant to the formation of the ACTSR distributions of that published application. Unfortunately, the disclosure of that document does not provide values for MZ, so the shape in terms of skewness and asymmetry cannot be determined for the distributions reported. A summary of the experimental disclosure in that document is basically as follows:                1. Use of a large volume of an inert medium (cyclohexane)        2. Use of a metal-alkoxide co-catalyst        3. Balancing rate of polymerization and rate of chain transfer such that they are of same order of magnitude via:                    a. Rate of addition of styrene            b. Very limiting amount of ethylbenzene relative to styrene            c. Moles of butyllithium initiator relative to ethylbenzene            d. Temperature (60-80° C. preferred)            e. Use of a Promoter, TMEDA                        4. Relatively Long Monomer Feed, controlled gradual addition of styrene, “starve feeding”.        5. Reactivation of “dead” polymer chains in a thermodynamic equilibrium to control shape of distribution.        6. Equilibration of chain transfer between dead and living polymer chains.        
EP 0 741 147 A1 contains 7 examples setting forth experimental details. Of these, only Example 1 entails an anionic chain transfer styrenic telomerization reaction (sometimes referred to hereinafter as “ACTST reaction”) in which a styrenic telomer distribution was formed without added diluent. That EBSTD had a DPn of 0.56, likely best described using an exponential probability density function, ƒ(xi)=1/βexp-Mi/β where β is a scaling parameter of that probability density function. The telomerization process of Example 1 was conducted in ethylbenzene as both the solvent and chain transfer agent. And in Example 1, the styrene was fed in a continuous controlled operation lasting 18 hours.
In the course of our investigation of bromination of toluene styrenic telomer distributions (TSTDs) formed from chemistry analogous to Example 1, in the absence of potassium t-butoxide cocatalyst using much faster feed rates as comparative Example 1 of EP 0 741 147 A1, but slower than the feed rates of this invention, these TSTD materials once brominated, did not provide the superior thermal color performance of the brominated flame retardants of this invention, nor did they provide the enhanced melt flow properties when used as flame retardants in high-impact polystyrene (HIPS) as compared to the brominated flame retardants of this invention. Such TSTD materials suffered from increased asymmetry reflected by broader distributions and significant skewness. A typical distribution is represented in FIG. 2. Note, the sequentially decreasing relative weight percent with increasing molecular weight of each individual telomer chain in the distribution shown in FIG. 2.
The other six Examples of EP 0 741 147 A1 entail using limiting quantities of the chain transfer agent ethylbenzene and a large volume of a diluent, cyclohexane. The process was designed to yield styrenic distributions having less than 10 wt % of components with molecular weight <350 g/mole. From an English translation of EP 0 741 147 A1, it was noted that the document makes several kinetic and thermodynamic arguments as to why the process could be made to approximate a Poisson distribution. These arguments are as follows:
“ . . . Counterion, activators, inflow rates and temperature should be chosen to ensure that the establishment of equilibria . . . takes place as rapidly as the chain growth . . . . If this condition is met, relative molar mass distributions are obtained that correspond approximately to the Poisson distribution with PDI=Mw/Mn=1+1/Pn where Pn is obtained from the quotient from the number of moles of monomer and the transfer agent after the complete and irreversible course of reaction. Otherwise, Pn=moles(monomer)/(number of moles transfer agent+number moles organic base) . . . ”
In the Detailed Description section presented hereinafter, three comparative examples based on Example 4 of EP 0 741 147 A1 are presented. We have found that this process yields ethylbenzene styrenic telomer distributions (EBSTD) best modeled by a Lognormal probability density function, written as ƒ(xi)=(Miσ√2π)−1exp−(ln(Mi)−μ)2/2σ2). Such a model is only in part contrary to the teachings of that document regarding formation of certain equilibria. Using that model and the values Mn=932, Mw=1500 reported for Example 4 of EP 0 741 147 A1, we predict a value for Mz of 2376 Daltons. Table 1 hereof presents reaction parameters reported or derived from experimental details reported in EP 0 741 147 A1.
TABLE 1Example #1234567Cyclohexane Diluent (ml)0155815581558155815581558g Cyclohexane/g Styrene00.360.360.960.960.960.36Mole Styrene/mole0.447.037.037.0914.1814.1818.86EthylbenzeneMole Styrene/mole0.020.391.171.182.362.361.05Ethylbenzene/hrMole Styrene/mole10.6710.6631.9832.0664.1232.0621.30Lithium/hrDPn0.567.4010.637.9419.5315.674.70Mn164876121293221371736596Mw262197004180015004830375033300MzNRNRNRNRNRNRNRPD1.6022.5034.501.612.262.1655.90Standard Deviation12740607013728239918704414σn = (MwMn−Mn2)1/2
From the experimental details of EP 0 741 147 A1, as presented in Table 1 above, it can be seen by comparison of Examples 2-7, that only Example 4 produced an anionic chain transfer styrenic reaction distribution (ACTSR distribution) having limited breadth (standard deviation) and small polydispersity. Minor changes in the relative feed rates or charges as reported, or both simultaneously, resulted in ACTSR distributions having very large standard deviations and having polydispersity that increase significantly, and in some Examples, astronomically. Thus, it can be seen that from such experimental details, a very narrow and limited process window for producing distributions with narrow breath, i.e., small standard deviation σn is provided.
Of the six Examples of EP 0 741 147 A1, only Example 4 has some molecular weight parameters or values that might indicate it would be useful for forming a brominated flame retardant. However, after preparing an analogous distribution formed from toluene instead of ethylbenzene, we have found that the high value for Mz characteristic of the process chemistry of EP 0 741 147 A1 renders the distributions of low utility and little interest. The impact of having the high Mz, and what it reflects, is best represented in FIG. 3. Characteristic of the product distributions of the process chemistry of EP 0 741 147 A1, is a very highly skewed and highly populated high molecular weight tail. Thus, though the material has a degree of polymerization of about 8, it has very significant levels (e.g., greater than about 25 wt %) of component telomer chains with molecular weights in excess of 2,000 molecular weight and still significant levels (e.g., greater than about 2.5 wt %) of component telomer chains with molecular weights in excess of 5,000 molecular weight, hence such a distribution loses the advantages gained from having short chain lengths and the ability to be brominated at levels in excess of 71 wt % bromine, cleanly and free of chain cleavage side reactions. What's more, the long chains contribute significantly to increases in the glass transition temperature, Tg, of the brominated distribution providing significantly reduced flow and impact properties when used as a flame retardant in high impact polystyrene resin applications (HIPS).
Much research has been conducted over the years in the search for new brominated flame retardants having superior properties, and such research has included a variety of very low molecular weight (number average DP<21) oligostyrene materials, none of which has achieved commercial success. Indeed, so far as is known, no one has ever produced brominated flame retardants having broad spectrum application with the combination of superior properties of the brominated flame retardants provided by this invention. As will be seen from the ensuing description, this invention provides novel robust toluene styrenic telomer distributions having a number average DP<6.5 with molecular weight distributions that are narrowly distributed with limited skewness and low asymmetry which distributions afford on bromination, the unique superior brominated flame retardants of this invention. Moreover, these novel styrenic telomer distributions and the brominated polymeric flame retardants can be produced on an economically attractive and industrially feasible commercial scale.